In order to use a different set other than identicon, replace &d=identicon portion of the URL with another supported set. For example, you can use retro set in which case the URL would look like: plain_url: "http://cdn.libravatar.org/avatar/%{hash}?s=%{size}&d=retro"

If your users are Office 365-users, the "GetPersonaPhoto" service can be used. Note that this service requires login, so this use case is most useful in a corporate installation, where all users have access to Office 365.